It is known that the exhaust gas after-treatment systems are generally equipped with one or more catalytic devices, which are provided for reducing the polluting emissions of the internal combustion engines. For example, the exhaust gas after-treatment system of a Diesel engine can be provided with a Diesel Oxidation Catalyst (DOC) and/or with a Lean NOx Trap (LNT) and/or a Selective Reduction Catalyst (SCR).
DOC is a catalytic device which contains catalysts, such as palladium and platinum, for reacting with hydrocarbon (HC) and carbon monoxide (CO) contained in the exhaust gas, in order to oxidize them into carbon dioxide (CO2) and water (H2O). LNT is a catalytic device containing catalysts, such as platinum, palladium, rhodium and absorbent such as barium based elements, which provide active sites suitable for binding the nitrogen oxides (NOx) contained in the exhaust gas, in order to trap them within the device itself.
SCR is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatonic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3) that can be obtained by urea (CH4N2O) thermo-hydrolysis and that is absorbed inside catalyst. Typically, urea is injected in the exhaust line and mixed with the exhaust gas upstream the SCR. Each catalytic device is generally characterized by a particular efficiency parameter, such as for example the efficiency of the oxidation reactions within the DOC, the nitrogen oxides storage capacity and conversion efficiency within the LNT, and the efficiency of the ammonia storage and reduction reactions within the SCR. Such efficiency parameter is often used by the engine control system for performing important actions. For example, the efficiency of the oxidation reactions within the DOC is an important parameter for the engine control system to control an Exhaust Gas Recirculation (EGR) system and to adapt particulate filter regeneration process (based on exothermal reactions).
The Exhaust Gas Recirculation (EGR) system is generally provided for routing back a certain amount of exhaust gas to the intake manifold of the engine. Such exhaust gas has the effect of reducing nitrogen oxides (NOx) emission, but has also the drawback of contemporaneously increasing the oxidizing hydrocarbon (HC) and carbon monoxide (CO) emission. Therefore, the DOC efficiency may be used by the engine control system to adapt the amount of exhaust gas which can be routed back to the engine.
The nitrogen oxides (NOx) storage capacity within the LNT is an important parameter for the engine control system to control the regeneration process of the LNT itself. Such regeneration process is generally provided for release and reduction of the trapped nitrogen oxides (NOx) from the LNT. Therefore, the LNT storage capacity may be used by the engine control system for effectively determining when the regeneration process is needed.
The efficiency of the reduction reactions and ammonia storage capacity within the SCR are important parameters for the engine control system to control the devices which are provided for injecting the reducing agent within the exhaust line upstream the SCR. In particular, the SCR efficiency may be used by the engine control system for determining the appropriate amount of reducing agent needed to effectively reduce the nitrogen oxides (NOx) within the SCR.
During the engine lifetime, the efficiency parameter of a catalytic device can be affected by progressive decrease, which is caused by aging effects. For this reason, many engine control systems are configured for estimating an aging index of the catalytic device, and for determining the efficiency parameter of the catalytic device in function of said aging index.
Conventional strategy for estimating the aging index are essentially based on temporal criteria, which for example correlate the aging index to the number of kilometers covered by the vehicle on which the engine is mounted. However, it has been found that the aging of a catalytic device is strongly affected also by the operating temperatures to which the catalytic device is subject. In particular, it has been found that a catalytic device at high operating temperatures ages faster than at low operating temperatures, to thereby resulting in a faster decrease of its efficiency parameter. Theoretically, it could be determined a maximum operating temperature, above which the efficiency parameter of the catalytic device decreases immediately at zero, and a minimum operating temperature, below which the efficiency parameter of the catalytic device does not decrease at all. The operating temperature of a catalytic device generally depends on the vehicle driving condition.
While the conventional strategies for estimating the aging index are typically calibrated on average driving conditions, the catalytic device can be subjected to higher operating temperature during urban driving cycles than during extra-urban driving cycles, for example when particulate filter regeneration is active. Therefore, if the vehicle is principally used in extra-urban driving cycles, the catalytic device is subjected to less aggressive particulate filter regeneration temperatures, so that the conventional strategies return an aging index which is higher than the real one. If, conversely, the vehicle is principally used in urban driving cycles, the catalytic device is subject to more aggressive particulate filter regeneration temperature, so that the conventional strategies return an aging index which is lower than the real one.
From the above it follows that the conventional strategies has the drawback of returning an aging index which often deviates from the real one. Such deviation involves an error in the determination of the efficiency parameter, which can cause the engine control system to perform a bad control of each process in which such efficiency parameter is implicated.
Accordingly, in view of the foregoing, at least one object is to solve, or at least to positively reduce, the above mentioned drawback with a simple, rational and inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.